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Explanation of how sound is detected - Succeed in Physical Science. Also refer to physics, ear, hearing, microphone, vibration, process, electrical signals, amplified, music, loud sounds, spy devices, laser, Ron Kurtus, School for Champions. Copyright © Restrictions

Detecting Sound Waves

by Ron Kurtus (19 February 2002)

Sound waves are detected by the fact that they cause objects to vibrate. Your ear and a microphone are common detectors of sound. They work by converting the sound wave into a signal and then amplifying and processing that signal.

Questions you may have include:

• How does sound cause vibrations?
• How is sound processed?
• What are some devices to detect sound?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

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Causes objects to vibrate

Just as a vibrating object creates sound, thus forming compression waves in air or some other medium, sound is also detected by the waves causing a back-and-forth vibration of some object in its path.

You can feel how sound can cause other things to vibrate by standing in front of some loudspeakers when music is being played very loud. You can actually feel the vibration on your skin and chest.

Loud sounds in a room can cause the windows and even walls to vibrate at the frequency of the waveform.

Vibration must be processed

The detection of sound waves requires transferring the vibration it causes into some sort of signal that can be processed and used.

Feeling the vibration of a wall when loud music is being played in the other room is detecting the sound, by changing the vibration into signals to your brain from your sense of touch. But that isn't very useful information.

Your ear or a microphone can convert the vibration into a signal, which can then be processed into a form that can duplicate or reproduce that sound.

The type of signal that the vibration creates is usually an electrical signal. Processing can almost duplicate the original sound, except for some distortions.

(See Reproducing Sounds for more information.)

Detectors

There are a number of devices used to detect sound. The most common are the ear and the microphone.

How the ear works

The ear has a small membrane called an eardrum. Sound causes to the membrane to vibrate, which in turn cause tiny hairs in your inner ear to vibrate, according to their designated frequency. Each hair sends an electrical impulse to the brain, where the signals are process and turned into the perception of sound.

(See Hearing and Sensing Sound for more information.)

How a microphone works

The most common mechanical detector of sound is the microphone. It has a membrane that is made to vibrate by the sound. That vibration is changed to electrical signals, which are then sent to a processor or electronic circuitry for amplification or such.

The electrical signal can then be sent to a loudspeaker to create sound at a greater volume, to a tape recorder, or to send out radio or TV signals.

(See Magnetism for more about how a microphone works.)

Spy devices

The secret agencies in various countries often try to spy on their opponents to gain information. They may try to "bug" an office by hiding tiny microphones in areas where they can listen in on conversations.

They have also devised some very clever ways of detecting sounds and conversations at a distance.

CIA and FBI

The American intelligence agencies have used lasers to detect conversations. From a distance outside, they would shine a small, invisible laser spot on a window of a building where secret conversations where being held. The reflected light off the window would be distorted due to the subtle vibrations of the glass from the conversations inside the room.

The device would detect the reflected light and convert it into electrical signals.

KGB

The Russian or Soviet Union secret service went one-up on this device. Since people were aware of the possibility of someone bugging their office or using a laser on a window to detect conversations in an office, they would often hold their secret conversations out in the park, away from any spy devices.

To spy on such conversations, the KGB developed a powerful telescope with a laser that could actually detect vibrations off a blade of grass near the people having the conversation.

Not ethical

Although it isn't ethical for the average person to spy on someone else's conversation, it does illustrate how sound causes vibrations that can be detected, processed and used.

Summary

Sound causes objects to vibrate, which is used to detect and process the sound. The ear and a microphone are common detectors of sound, although there are also some clever devices to detect sound at a distance.

Answers to Readers' Questions

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Resources

The following resources provide information on this subject:

Websites

The Power of Hearing - From PhysicsWeb magazine

Mircophones - From HyperPhysics

Physical Science Resources

Books

Top-rated books on Sound

Miscellaneous

Mini-quiz to check your understanding

1. How could a deaf person sense sounds?

2. What is the purpose of processing the signal created by a detector from a sound wave?

3. Why would you cup your ears with your hands when trying to hear faint sounds?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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Detecting Sound Waves